Electrons bound inside a solid can be promoted to vacuum by one or a combination of three major techniques: thermionic emission, field emission, or photoemission. Each method has advantages and disadvantages.
The most common method is thermionic emission, a process that requires heating the cathode (electron-emitting) material to high temperatures. The high temperature creates a fundamental limit on the lifetime of the cathode.
Field emission can occur at low temperatures, but the need to create a very high electric field at the emission site and issues associated with the high electric field limit the application of field emission sources.
The third major method of producing vacuum electrons is photoemission. Advantages of photoemission include the ability to control the time and location of the emission by changing the time and location of the incident light. For example, photoemission is often used to produce electron pulses too short to be produced by other means, typically less than about 1 nS. At normal intensities the energy of the illuminating photons hv must exceed the minimum required energy Emin to remove electrons from the emitting material. For a metallic surface, Emin is the work function of the surface. The range of energies produced is hv-Emin. Normally it is necessary to make hv significantly larger than Emin, since the value of Emin varies with location across the surface and with the chemical condition of the surface.
Chemically stable metals typically have Emin near or above 4.5 eV, so hv must be greater than 4.5 eV, which is in the ultraviolet. Typically ultraviolet light sources are more expensive and more difficult to work with than visible sources. The energy Emin can be reduced by coating some surfaces with special materials such as alkali metals and their oxides or salts (such as Cs, CsO, or CsI). However, these materials are chemically reactive, making then difficult to work with and limiting the useful lifetime of photo-cathodes that use such coatings.
Many devices require electron beams that can be collimated or focused to a small spot. These qualities are limited by the size of the electron source and the kinetic energy transverse to the beam direction produced by the electron source. The product of the source size and transverse energy is proportional to the emittance. Equal emittance, and thus equal beam quality, can be achieved by trading off emission area and transverse energy.
Diamond surfaces terminated with hydrogen have negative electron affinity (NEA), such that electrons with energies at or above the conduction band minimum can be emitted to vacuum without gaining additional energy. This property allows efficient electron emission for electrons in the conduction band when those electrons encounter the NEA surface. The fraction of electrons that encounter the surface is enhanced or diminished by negative or positive electric field in the diamond between the electron and the surface.
Promoting an electron from the diamond valence band directly to the conduction band requires adding at least slightly more than the band gap energy (5.5 eV) to the electron, requiring light with wavelength λ<220 nm. Light with energy less than 5.5 eV can also promote electrons to the conduction band via less efficient two-step or multi-step processes involving energy states in the bandgap caused by crystal lattice imperfections such as impurity atoms or carbon vacancies. This absorption rapidly becomes weaker with longer wavelength, such that the ratio of emitted electrons to incident photons (the quantum efficiency or QE) becomes lower for longer wavelengths. Most reported measurements of photoemission from diamond showed undetectable emission current for light wavelengths λ>275 nm. One report showed a detectable emission threshold near 3.0 eV (413 nm), however no repeatable process was described to produce the low energy emission.